75 



the Hamilton period, the water over the present Erie Count}- 

 was deeper and purer than it was in the region of the 

 present Genesee valW, where the Marcellus conditions con- 

 tinued into, and through the Lower Hamilton time. Farther 

 east, /. e. nearer to the source of supply of the fragmental 

 material, the water was still shallower and sands accumu- 

 lated, the increase in the coarseness of which, was propor- 

 tionate to their nearness to the old shore-line. It is an 

 interesting fact, that in these near-shore formations we 

 have a littoral fauna, consisting mainly of gastropods and 

 pelecypods, brachiopods, which are the predominating forms 

 in the deeper western waters, being almost entirely absent. 



When we compare the fauna of the Hamilton shales in the 

 region about Eighteen Mile Creek with that of the corre- 

 sponding shales in the Genesee Valley, we will be impressed 

 by the numerical preponderance, in species and individuals, 

 of the life in the more western area. This indicates more 

 favorable conditions in the Eighteen Mile Creek region, and 

 consequently a greater luxuriance of life. The subsidence of 

 fift\^ feet which occurred in this region during the deposition 

 of the Hamilton shales was not uniform throughout, as is 

 indicated by the alternations of coarser and finer material, 

 and to some extent also by the variation of the faunas, and 

 the fluctuation in the number of fossils in the various beds. 

 That these local topographic faunas were influenced by local 

 topographic changes, will be conceded, even though the 

 precise changes can not be determined, just as the changes 

 on modern shores can not always be determined, though 

 their effects in the disappearance or reappearance of faunas 

 may be quite marked. It is certain that many species, which 

 flourished at the beginning of the Hamilton epoch, soon 

 disappeared, and did not again occupy this region. An 

 example is the great Nautilus magister, which flourished at 

 the opening of the Hamilton epoch, but the remains of which 

 are only found in the lowest true Hamilton bed, showing 

 that the species disapjjeared at the end of the time occupied 



